Major changes have taken place within the heating, ventilating and air-conditioning ("HVAC") industry as a result of new regulations enacted to address certain indoor air quality ("IAQ") problems, which can result in what has been commonly referred to as "sick building syndrome." Several years ago, the American Society of Heating Refrigeration and Air-Conditioning Engineers ("ASHRAE"), the HVAC industry's trade organization, issued a new standard (IAQ Standard 62-1989), which significantly increased the recommended quantity of outdoor air to be drawn in and processed by an HVAC system installed to operate with a commercial facility. Since that time, this standard has been adopted by most major building codes. Currently, ASHRAE is in the process of revising the 62-1989 standard, altering its presentation format and revising and expanding numerous aspects of the standard. Among the most significant changes include new methods for calculating outdoor air quantities (i.e., the amount of outdoor air processed by the HVAC system in a given period of time), requirements for space humidity control, and proof that the required outdoor air quantities are being delivered to the occupied space on a continuous basis.
Most (greater than 75%) of all commercial buildings use packaged heating and air-conditioning equipment. Typically, this equipment consists of a compressor, various fans, filters, dampers, and associated devices. This equipment is easy to install, performs well if used as designed, and is very cost effective. However, the new IAQ requirements presented by ASHRAE tax the ability of such packaged equipment to perform as intended. Such equipment was conventionally designed to process only a relatively small quantity of outdoor air (e.g., approximately 15% of the total air processed, the bulk of the total being re-circulated air). In addition, the system fan is generally operated on an intermittent basis, providing outdoor air to the conditioned space only when the heating or cooling source is energized. In most cases, following the new IAQ requirements results in providing a much higher proportion of outdoor air compared to recirculated air on a continuous basis than was heretofore necessary. If a conventional packaged unit is operated to comply with the outdoor air requirements of the ASHRAE 62-1989 standard, the resulting humidity control within the space is frequently less than desired, especially at partial load conditions.
As is known, a solution to meet the new IAQ requirements is to precondition the outdoor air entering the packaged HVAC units with a device commonly known as an air-to-air energy exchanger. A preferred air-to-air energy exchanger, a total energy recovery wheel, is described in Fischer et al. U.S. Pat. No. 4,769,053 (also assigned to the assignor of the present invention). Using such devices, air that is exhausted from the conditioned space is first passed through an air preconditioning module containing the air-to-air energy exchanger, which typically recovers a high percentage (e.g., up to 80%) of the total energy contained in this exhaust air stream and uses this recovered energy to pre-cool and dehumidify the outdoor air during the cooling season and to preheat and humidify the outdoor air during the heating season. Thus, the addition of an air preconditioning module permits a packaged HVAC unit to process approximately four times more outdoor air on a continuous basis than it otherwise could, thereby allowing the system under the new ASHRAE requirements to maintain indoor temperature and humidity conditions similar to those maintained with the same packaged HVAC unit without the use of an air preconditioning module before the new ASHRAE requirements.
The supply and return air quantities to and from the packaged HVAC units (either standing alone or when used in combination with an air preconditioning module) are easily measured in the duct work extending from the HVAC unit into the conditioned space using conventional methods. One such method, which may be used when the packaged system is initially balanced for airflow, is to use a conventional pitot tube or hot wire anemometer traversing the ductwork. Another common approach is to use an airflow station/hood to determine flow on a register-by-register basis. These procedures are easy to implement and reasonably reliable. However, principally due to space restrictions, these two conventional methods are not easily applicable to the task of measuring the quantity of outdoor air provided to, or exhaust air purged from, the conditioned space served by packaged HVAC equipment. Typical HVAC packaged systems, including those coupled with an air preconditioning module, do not allow adequate access to install and operate conventional airflow measuring devices to quantify the amount of outdoor (or exhaust air) being delivered by the unit. For example, the outdoor air quantity, which is typically set by manually adjusting the inlet hood/damper combination connected to the return air section of the packaged system, cannot readily be measured because the amount of space available for taking such measurements is too small or the inlet velocity too uneven to effectively apply accurate airflow measuring equipment.
Several alternative methods are currently used to estimate the amount of outdoor and exhaust air entering and exiting a conditioned spaced. For example, one such method is to estimate the outdoor airflow by measuring the average velocity of the air into the hood and then multiplying this velocity times the open area of the hood. The error using this approach is considerable, that is, the likelihood that the correct value will be determined a high percentage of the time is low.
Furthermore, measuring the outdoor air quantity on a continuous basis presents additional difficulties and is seldom done with packaged HVAC systems. Although conventional airflow stations are available for this purpose, to apply these stations to a packaged HVAC unit requires an additional plenum to be added to the system, resulting in a significant increase in the cost and size of the HVAC unit. The addition of such a plenum becomes more cumbersome if the system includes an air preconditioning module (as described above) to satisfy the recent IAQ performance requirements imposed on the system.
Thus, there is a continuing need for an efficient, accurate, and cost effective method to measure the amount of outdoor air (i.e., fresh or non-recycled air) delivered to a conditioned space, especially when such outdoor air is processed through an air preconditioning module operating in conjunction with a conventional packaged HVAC unit. Preferably, the method would also provide a means to measure the volume of exhaust air emitted from the conditioned space (i.e., air bled or removed from the system and, for example, exhausted to the outdoors) through the air preconditioning module and a means for continuously recording the volume of outdoor and exhaust air.